Study of Equilibrium Properties of Cr(III)/Polyacrylamide Gels by Swelling Measurement and Equilibrium Dialysis

Abstract

Summary In observations of the long-term properties of a series of Cr(III)/polyacrylamide (PAAM) gels, the gels either underwent syneresis upon aging or swelled in contact with excess brine. Both syneresis and swelling can substantially change the volume and properties of a gel placed in a formation and therefore influence the effectiveness of a crosslinked-polymer treatment. Efforts were made to develop an understanding of these phenomena and to describe them in terms of the gel's physical and chemical states. A long-term gel's physical and chemical states are characterized by two parameters, effective crosslinking density and chromium density, determined by equilibrium swelling and equilibrium dialysis, respectively. Swelling and syneresis properties were correlated to the effective cross-linking density described in polymer network theory. A model based on Flory and Hermans' swelling equations was developed to calculate the effective crosslinking densities of gels prepared from solution. Attempts were made to relate the swelling and syneresis properties to the compositions of gel systems to allow prediction of long-term stability of a gel based on its composition. An analysis of the amount of chromium that reacted with the PAAM was made by successive equilibrium dialyses of the gel followed by chromium analyses of dialysates by atomic and visible absorptions. The overall fractional conversion of chromium in the gelation was between 0.39 and 0.70. The equilibrium conversion of Cr(VI) to Cr(IH) ranged from 0.65 to 0.85. The effective crosslinking density obtained from the swelling experiment was <5 % of the chromium density for all samples studied. A substantial fraction of reacted chromium is suspected to be present in the gel matrix either in the form of intramolecular crosslinks or attached to only one polymer segment.